Concrete is a conglomerate of aggregate (such as gravel, sand, and/or crushed stone), water, and hydraulic cement (such as portland cement), as well as other components and/or additives. Concrete is generally fluidic when it is first made, enabling it to be poured or placed into shapes, and then later hardens, and is never again fluidic, in the general sense. Typically, moisture present in concrete is basic (that is, has a high pH). Concrete also typically includes alkali materials supplied by the cement, aggregate, additives, and even from the environment in which the hardened concrete exists (such as salts placed on concrete to melt ice).
Siliceous minerals can be present in certain aggregates found in concrete and mortars. Silica which is present in aggregates used to make concrete and mortars is subject to attack and dissolution by hydroxide ions present in basic solutions. Generally, the higher the pH (i.e., the more basic the solution), the faster the attack.
Different forms of silica show varying degrees of susceptibility to this dissolution. If there is sufficient alkali metal ion also present in this solution (such as sodium or potassium ions), the alkali metal ions can react with the dissolved silica and form an alkali-silica gel. Under certain conditions, the resultant alkali-silica gel can absorb water and swell. The swelling can exert pressures greater than the tensile strength of the concrete and thus cause the concrete to swell and crack. This process (hydroxide attack of silica, followed by reaction with alkali such as sodium and potassium) is referred to generally in the art as "alkali-silica reaction" or "ASR".
In the late 1930s and early 1940s, Stanton first identified the expansion and deterioration of portland cement-base concrete caused by ASR in the western part of the United States. T. E. Stanton, "Expansion of Concrete through Reaction between Cement and Aggregate," Proceedings of the Am. Soc. of Civil Engineers 66: 1781-1811 (1940). Since then, numerous structures have been reported as suffering from ASR in concrete around the globe.
ASR can weaken the ability of concrete to withstand other forms of attack. For example, concrete that is cracked due to this process can permit a greater degree of saturation and is therefore much more susceptible to damage as a result of "freeze-thaw" cycles. Similarly, cracks in the surfaces of steel reinforced concrete can compromise the ability of the concrete to keep out salts when subjected to de-icers, thus allowing corrosion of the steel it was designed to protect. Although rare, ASR can also cause the failure of concrete structures.
Since the discovery of ASR, researchers around the world have been trying to control this detrimental attack on the concrete structures. Prior attempts to control ASR include, for example, using cement with very low alkali content, non-reactive aggregate, and pozzolanic materials such as fly ash, silica fume, ground blast granulated furnace slag, zeolite minerals, thermally activated clay, and the like.
Lithium-based compounds have been shown to be effective in ASR inhibition by introducing these chemicals into concrete or mortar mix compositions. W. J. McCoy and A. G. Caldwell, "New Approach to Inhibiting Alkali-Aggregate Expansion," J.Amer.Concrete Institute, 22:693-706 (1951). However, this requires introducing the lithium-based compounds in the concrete or mortar mixture and does not address the problem of controlling or remediating ASR in existing hardened structures.
U.S. Pat. No. 4,931,314 is directed to a process for preventing a hardened cementitious material from deteriorating or for repairing a deteriorated cementitious material due to ASR. In this process, a cement paste, mortar or concrete with lithium nitrite incorporated therein is applied to an existing concrete system and allowed to harden. While stated to be effective in delivering lithium into the concrete structure, this process is time consuming and inconvenient or impractical because lithium nitrite is incorporated in a second cementitious layer applied to an already existing structure.
Expansion due to ASR in mortar bars and concrete prisms has reportedly been reduced by soaking the specimens in solutions of LiNO.sub.2. See Y. Sakaguchi, et al., "The Inhibiting Effect of Lithium Compounds on Alkali-Silica Reaction," Proceedings, 8th International Conference, Alkali Aggregate Reaction, Kyoto, Japan: 229-234 (1989). However, soaking existing concrete structures in a LiNO.sub.2 solution is difficult and not practical. Further, the effectiveness of this process is questionable in view of other studies indicating that solutions of other lithium compounds show very little penetration into existing hardened concrete structures, as described below.
The Strategic Highway Research Program (SHRP) publication SHRP-C-343 investigated a method of mitigating ASR in existing concrete by spreading lithium hydroxide solutions on the surface of the concrete. Difficulties were encountered, however, in effectively delivering the materials into concrete. SHRP also demonstrated that ASR can be reduced by soaking cementitious specimens in aqueous solutions of lithium carbonate, fluoride, and hydroxide. Again, however, such techniques are impractical in treating real life existing damaged concrete structures. Further, the effectiveness of this technique is questionable in view of the reported difficulties in effectively delivering lithium hydroxide into concrete by applying the solution onto an existing structure.
Because of these and other difficulties, processes have been developed to electrically drive lithium ions into concrete to mitigate ASR in concrete with conductive metals imbedded in the concrete. While lithium can be effectively delivered into a concrete structure electrically, such techniques typically require specialized equipment, careful maintenance and control of the electrolyte solution and equipment, and readily available utilities. In addition, these processes typically work only with concrete that contains reinforced steel (which is typically not found in highways) and can result in penetration only to the steel. Further, these processes may not work if the steel is coated (i.e., epoxy coated), which is increasingly common.
Problems can also arise in the repair of concrete materials when applying solutions onto and into large masses of concrete that may have substantial cracks, particularly when the bottom surface is not accessible because it is on or below another surface, such as the ground. Materials can pass through such cracks without being in contact long enough to penetrate the concrete mass itself.